Many of the automatic transmissions of automobiles are constituted with a torque converter, a wet type clutch, gears, bearings and a hydraulic control mechanism. In the torque converter, the medium which works for transfer of power is an automatic transmission fluid. The engine torque is transferred to the transmission via the automatic transmission fluid.
Energy saving is greatly required in recent years, and an increase in the efficiency of power transfer is required for the transmission. To satisfy the requirement, a lock-up clutch effective for improving the fuel economy is disposed at the inside of the torque converter in the automatic transmission. The lock-up clutch transfers the engine torque directly to the automatic transmission mechanism in accordance with the driving condition of the automobile. The efficiency of the torque converter can be improved by switching between the driving through the torque converter and the direct driving at the optimal times.
However, the lock-up clutch has not been used in the low speed range since the ride becomes poor due to fluctuations in the torque of the engine when the lock-up clutch is used in the low speed range. Therefore, in the low speed range, a loss in the power transfer arises during the torque transfer between the rotation of the engine and the rotation of the transmission by the torque converter. To decrease the loss in the power transfer and improve the fuel economy, a slip control method for absorbing the fluctuations in the torque of the engine during the use of the lock-up clutch in the low speed range is recently introduced. However, the slip control method has a problem in that abnormal vibration of the body, which is called shudder, takes place at the faces of friction in the lock-up clutch. To prevent the shudder, a lubricant composition which exhibits an improved μ-V property (μ: the friction coefficient; V: the slip speed) so that the friction coefficient increases as the slip speed increases and, therefore, exhibits an excellent property for anti-shudder, is required.
The transmission has a wet type clutch, and transmission shock takes place when the friction property of the clutch in the transmission is poor. Therefore, a lubricant composition exhibiting an excellent friction property is required so that the transmission shock is decreased.
Esters of phosphoric acid, amides of fatty acids, esters of fatty acids and amines have heretofore been used as the friction modifier to improve the property for anti-shudder and the friction property. However, friction coefficient decreases and the transmission torque capacity becomes insufficient when the friction modifier is added in a great amount. Development of a lubricant for transmissions which can maintain the mechanism of anti-shudder without decreasing the transmission torque capacity is desired.
In the automatic transmission, a one-way clutch is used for the stator and the transmission clutch in the torque converter in many cases. The one-way clutch is fixed by the friction force between metals of balls and rollers disposed at the inside, and the rotation in the direction opposite to the direction of rotation is prevented. However, when the friction modifier is added in a great amount, there is the possibility that the friction coefficient decreases, and the one-way clutch makes slipping. Therefore, a lubricant for transmissions which can maintain the friction coefficient between metals is necessary.
As the lubricant composition for automatic transmissions, for example, a composition containing a bisimide compound having a hydrocarbon group having at least 5 carbon atoms, which can improve the property of anti-shudder in the transmission for automobiles equipped with the slip control mechanism even when the lockup mechanism is used in the low speed range, is disclosed (for example, Japanese Patent Application Laid-Open No. Heisei 9(1997)-202890). A lubricant composition containing a bisimide compound having a hydrocarbon group having 8 to 30 carbon atoms and an ashless dispersant modified with boron, which can maintain the property of anti-shudder in the low speed range and can prevent cleavage of the clutch, is disclosed (for example, Japanese Patent Application Laid-Open No. 2001-288489). A lubricant composition containing a bisimide and a monoimide having a hydrocarbon group having 8 to 30 carbon atoms, which maintains the property of anti-shudder in the low speed range and exhibits a great torque capacity and an excellent transmission range of the wet type clutch, is disclosed (for example, Japanese Patent Application Laid-Open No. 2002-105478). However, since these lubricant compositions do not exhibit sufficient stability, there is a limit for maintaining the property of anti-shudder. Therefore, a lubricant additive and a lubricant composition which exhibits a more excellent property of anti-shudder is strongly desired.